Hi M.,
My daughter who is also 2.5 was diagnosed a week ago with a UTI si I definitely relate to what you are experiencing. She is fully toilet trained and that unfortunately is probably how she got it as she is very independent and likes taking herself to the loo and "wiping". It's so easy for girls to get them (although my 4 year old has never had one). I need to make sure she lets me know when she has done a poop and lets me assist her as I can reinforce wiping from front to back.
My daughter is due to finish her course of antibiotics in a couple of days and I too, am returning to the Dr just to do a test to make sure that the treatment was successful. I took her to the Dr. strongly suspecting a UTI as she was crying when she urinated, actually wet her pants once which was very unusual for her, as well as going to the loo with significant increased frequency, I knew something was wrong.
Cranberry juice apparently has properties that help keep UTI's at bay and my Dr suggested I give my daughter a small amount each day. Have a look around on the net and see what info you come up with.
Make sure she drinks lots of water to keep her system flushed, and if she is toilet training, that she knows how to wipe correctly (a long time coming in my experience!). Give her some yoghurt to add some good bacteria to her system as well to help with being on antibiotics.
Give yourself a hug!
Cheers,